In general, high-voltage electrical insulating materials for use as insulators and bushings for power transmission lines or the like are of porcelain (ceramics) or glass. Since these insulators are heavy and liable to breakage due to a lack of impact resistance, they require careful handling and impose a burden to workers. In a pollutive environment as in seaside areas and industrial areas, there is a tendency that dust, salt and mist in particulate form attach to the surface of high-voltage electrical insulators, causing leakage currents and dry band discharge leading to flashover failure.
In order to eliminate the drawbacks of porcelain and glass insulators, a number of proposals were made. For example, U.S. Pat. No. 3,511,698 discloses a weathering resistant high-voltage electrical insulator comprising a member of thermosetting resin and a platinum catalyst-containing organopolysiloxane elastomer. JP-A S59-198604 proposes a one-part room temperature curable organopolysiloxane composition which is applied to the outer surface of an electrical insulator of glass or porcelain so that the electrical insulator may maintain its high insulating properties even in the presence of moisture, air pollutants, ultraviolet radiation and other outdoor stresses.
JP-B S53-035982 discloses that a silicone rubber composition with improved electrical insulation is obtained by heating a mixture of an organopolysiloxane capable of heat curing into silicone rubber and aluminum hydroxide at temperatures above 100° C. for more than 30 minutes. JP-A H07-057574 describes that the blending of a methylalkylsiloxane fluid in silicone rubber is effective for providing contact angle recovery with time and preventing flashover failure.
However, the silicone rubber materials used in the prior art techniques mentioned above are not yet fully satisfactory in high-voltage electrical insulation. They must be loaded with large amounts of aluminum hydroxide in order to improve the electrical insulation. Aluminum hydroxide, however, is so hydrophilic that it may degrade electrical properties in a humid environment. Solutions to this problem include surface treatment of aluminum hydroxide as disclosed in JP-A H08-259820, and aluminum hydroxide bearing a specific amount of alkenyl group on surface as disclosed in JP-A H11-152408. Also JP-A H11-012470 proposes an addition cure silicone rubber composition having an organopolysiloxane resin added thereto.
These compositions, however, still suffer from problems. On use in acidic industrial areas, high-voltage applications, or serious pollution areas, corona discharge can occur to form acid. This often invites degradation of silicone rubber.